Enter a diameter to see matching threads. Add pitch or TPI to narrow it down.
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Thread identifier
Measure the outside diameter of the bolt or the crest-to-crest pitch (or count threads per inch), and ThreadRef ranks the closest standard threads. Tap any result for its full drill, clearance and torque reference.
How to measure a thread
Diameter: close your calipers on the crest (outer edge) of the threads - that is the major or outside diameter. For metric it is roughly the size number (an M6 is about 6 mm across).
Pitch / TPI: metric threads are quoted as the millimetre distance from one crest to the next (the pitch). Inch threads are quoted as threads per inch (TPI) - lay a rule along the thread and count the crests in one inch, or use a thread pitch gauge.
Some pairs are separated by diameter and some only by pitch - M8 and 5/16" sit 0.062 mm apart, while M5 × 0.8 and #10-32 UNF have pitches six microns apart. The guide on identifying an unknown thread covers the measurements, the traps and the confusable pairs; see also all guides.
The ranking uses the standard major diameters and pitches in ThreadRef's database; it weights pitch more heavily than diameter because pitch is what separates a coarse thread from a fine one at the same size.